I have been thinking about a discusion on Hacker News yesterday about which new programming language people want (or need) to learn. While I definitely enjoy learning new programming languages by writing small applications, I think that I personally get a better productivity boost by reviewing languages and tools I already use.
In the last year I have mostly used Ruby and Clojure for my work. In the last few months I have read two books on Clojure and one on Ruby. Sort of: the more you know, the more you can learn and more deeply understand something.
Recently I reviewed the commands and read another tutorial for the screen utility that I have been using almost everyday for three years. Well worth the time.
I do a lot of work on remote servers and emacs is not always installed so I have also used vi for years. This morning, I saw a reference to learning vim by using vimtutor and spend 20 minutes working through the complete tutor program (learn by doing: you edit the tutorial as part of the tutorial). Time very well spent.